1. Field of the Invention
The present invention relates to a method of crystallizing an amorphous silicon layer by simultaneous dehydrogenation and crystallization using a laser beam having a predetermined profile.
2. Discussion of the Related Art
An amorphous silicon layer may be crystallized when treated with energy from a laser or the like, during which process the silicon layer melts and subsequently crystallizes when cooled. Crystallization occurs via a grain growth process where small crystal seeds which was not melted by the laser energy grow to form large crystals. When a plurality of seeds are present at different locations of the unmelted silicon, the seeds grow to form polycrystalline silicon.
Each grain of the polycrystalline silicon formed by this process has its own boundaries. When polycrystalline silicon is used as the channel region of a semiconductor device, the charge carrier mobility is typically low due to the grain boundary effect arising from the carriers having to pass through the boundaries between the grains.
In a liquid crystal display (LCD) device having thin film transistors (TFT) formed of low temperature polycrystalline silicon, active layers of TFTs are formed by depositing an amorphous silicon layer by plasma enhanced chemical vapor deposition (PECVD) and by subsequently annealing or crystallizing the amorphous silicon layer using a laser annealing technique.
A first prior art method comprises depositing an amorphous silicon layer using PECVD and subsequently crystallizing the amorphous silicon layer using laser annealing. The PECVD process results in an amorphous silicon layer containing about 15% hydrogen. Dehydrogenation is carried out by thermally treating the hydrogen-containing amorphous silicon layer at a temperature over 400.degree. C., before the laser annealing step. The thermal dehydrogenation process requires additional equipment such as a furnace or the like and typically takes about five hours. As a result, productivity is low and the cost is high. Moreover, the thermal annealing treatment using a furnace causes damages to metal structures, such as hill-lock and the like, in semiconductor devices where a metal layer lies below the silicon layer.
In a second prior art method, the amorphous silicon layer is formed by low pressure chemical vapor deposition (LPCVD) and has relatively low hydrogen content. The amorphous silicon layer is laser-annealed without a dehydrogenation process. This method results in silicon layers having a smooth surface because of the low hydrogen content of the amorphous silicon layer, but suffers from low productivity. In addition, the glass substrate used in the semiconductor device is deformed due to the relatively high temperature during the LPCVD process (over 500.degree. C.). Thus, new equipment and techniques are necessary for forming amorphous silicon layers at low temperatures and having low hydrogen content.